1. Field of the Invention
The present invention relates to a method for sterilizing a container which is ready to be filled. More specifically, the invention relates to a method for sterilizing a closed container which is ready to be filled, the container being in the form of a homogenous flat first board and a non-homogenous second board with substantially plane insides and having a space between the first and the second board.
2. Description of the Related Art
It has for a long time within the industry for packaging of foods been a need for a fast method for sterilizing a package material without using chemicals. Furthermore, it would often be of great importance if a sterilizing equipment could be avoided in the actual filling machines, i.e. that the containers could be sterilized in advance in connection with the manufacturing of the material. If premanufactured containers were accessible in large numbers at different places for filling of foods in filling machines, these machines could then be manufactured at a lower price and the investment costs would be lower since certain precautions for aseptic filling probably could be minimized. In addition, such machines can result in lower operational costs which by itself is not a minor advantage.
Package materials are presently sterilized by a treatment with steam, steam/condensation or hydrogen peroxide. Depending on the type of package material the inside of a container can be sterilized with hydrogen peroxide, in liquid or gaseous phase, with ultraviolet light, if necessary in combination with hydrogen peroxide, or with ultraviolet, infrared and visible light in the form of what is called "Pulsed Light". However, completely sealed containers can not be sterilized in this way. Thus, when sterilizing package materials trials have recently been performed in order to utilize beta or gamma radiation for the deactivation and/or killing of microorganisms and viruses. Usually beta radiation (exposure to electrons) is preferred since this type of sterilization is less hazardous and not that costly to produce.
However, air (oxygen) is converted to ozone when exposed to electrons. Ozone is a strong oxidant for organic substances, and ozone present in containers in turn reacts with the inside of the package material, an undesired residual flavor being obtained from such reaction products. The formation of ozone thus results in a product limitation, i.e. it will be more difficult to pack sensitive products. Such products are in principle all those which contain water; the more water in the product the more off taste producing compounds are solubilized from the plastic material. The presence of a residual flavor in the product might be avoided by improving the quality of the plastic in the material. However, materials with good ozone resistance, such as for example polyurethane materials, are expensive and furthermore not always applicable as a package material for specific purposes.
It is known to irradiate the inside as well as the outside of a not yet sealed package material with an electron gun (electron accelerator). It is also known to sterilize a container which is ready to be filled by means of exposure to electrons, the package under rotation being exposed to an electron beam from a window of said electron gun. The energy of the electron beam is adapted in such a way that the electron beam all the time penetrates that wall of the container which is facing said window and reaches the opposite inside wall of the container. However, with this type of sterilizing completed containers the energy of the electron beam must at all times be adapted to the geometry of the container as well as the thickness of the package material, i.e. unless the container is not completely symmetric a uniform radiation dose will not be received on the inside of the container. This means that during the irradiation the energy either must be increased or varied, which in turn results in a more costly procedure and/or an altogether abundant use of energy. At the same time it is important that an overdose is not achieved which can result in impaired packaging properties and what is called "off-flavor".